In wireless networks, resource restricted devices, including energy-restricted devices such as energy-harvesting devices, can be used. Such devices are heavily restricted in the available amount of energy, which limits their offered functionality and influences the network operation, commissioning and maintenance.
One example of such technology is the evolving ZigBee Green Power (GP) standard. A ZigBee Green Power Device (ZGPD) is a resource restricted device which may be powered by energy harvesting and which may not have a battery or which may only have a small storage capacity. Thus, a resource restricted device can transmit and/or receive only at unscheduled opportunities. One example of such a resource restricted device is a battery-less switch that can only transmit for a short time once it has been actuated by a user, but has no reception capability. Another example of a resource restricted device is a battery-less switch that can receive for a short time once it has transmitted a signal upon actuation by a user. Yet another example of a resource restricted device is a periodically-reporting sensor, harvesting energy from its environment, e.g., by means of a photovoltaic cell, with or without reception capabilities. If an energy-restricted device is out of range of the device it is configured to control (the device to be controlled referred to as destination device), an intermediate device (referred to as a proxy device) is used for forwarding information to the destination device. Generally, any device within a network that has forwarding capability may act as a proxy device, including a destination device. The wireless links between the proxy device and the resource restricted device may appear and disappear during network lifetime, e.g., due to changes in propagation conditions or in relative location of devices, and/or due to devices being added and removed. For system security and performance reasons, the proxy devices may be required to only forward for resource restricted devices they have a table entry (i.e. proxy table entry) for, e.g., to be able to perform freshness and/or security check (authentication, decryption), direct the message to appropriate destination device(s), etc. For communication reliability, more than one proxy device may be used for forwarding information on behalf of one resource restricted device.
The above described concept, e.g. the concept of ZigBee Green Power, was designed having home networks in mind: with limited number of devices, rather sparsely distributed, and self-organizing (capable to operate in the temporary absence of any of the devices). The concept may generally be used in managed network, where any self-configuration or -discovery is disabled, and management functionality is defined, which can be performed by a central device or shared by a number of devices, taking responsibility for the proper configuration and operation. However, the offered solutions do not scale well in large, dense networks, especially the proxy device assignment, e.g. proxy table entry management may become critical for large, dense networks.
International patent application WO 2013121339 A2 describes several techniques for managing the contents of proxy tables in ways that optimize performance, latency, and reliability for communication networks, such as Zigbee Green Power networks, while ensuring a certain number of early-acting proxies per restricted device. An entity is allowed to keep a required number of proxies per resource-restricted device by removing or adding proxy table entries.
The ZigBee GP specification defines two ways of managing the proxy device assignment, e.g. managing proxy table entries: a global and local approach.
In the global approach, management functionality performed by, e.g. a central device, e.g. a Commissioning Tool (CT), a concentrator, or a destination device or performed by a number of devices, e.g. sink devices, is responsible for managing the proxy devices forwarding on behalf of a particular resource restricted device, by directly configuring their proxy tables. Automatic configuration and discovery by the proxies themselves may be switched off. This approach may require analysis of network topology, signal propagation characteristics, etc. Furthermore, management functionality intervention is required not only during commissioning, when each proxy table is initially set up, but also during operation, since propagation conditions may change when a resource restricted device, a destination device or a proxy device is moved, removed or added.
In the local approach, upon creation of a pairing between a resource restricted device and a destination device, the destination device just broadcasts a GP Pairing command, and all proxy devices with an empty proxy table entry may add a corresponding entry. If a proxy device does not know a resource restricted device it is receiving frames from (i.e. has no table entry for that resource restricted device), it can discover the resource restricted device by broadcasting a GP Pairing Search command and/or a broadcast GP Notification command. While this works in small, sparse networks, such a self-organizing approach will not scale up to large scale dense networks such as for instance professional lighting applications. Such a professional lighting application may be used to control the light in a plurality of rooms located for instance in a floor of an office building. Assuming twenty rooms on a wing of an office floor, each equipped with only two lamps (which is on the low side), and only one resource restricted device in form of a switch per office room of say 3 by 3 meters (again this is on the low side), there will be around forty proxy devices within the wireless communication range of each of the resource restricted devices as well as nineteen other resource restricted devices competing for the same proxy table space and air time. The minimal requirement made by the Green Power v1.0 standard (ZigBee document 095499r26) is for a proxy device to have ten proxy table entries. Having more entries increases the memory usage, since each entry can be several tens of bytes long and the Green Power v1.0 standard (ZigBee document 095499r26) requires it to be stored persistently across device power cycles. The resulting network thus has to deal with a large number of proxy devices per resource restricted device, resulting in very high traffic inter alia due to redundant signal forwarding, with a large number of discoveries per resource restricted device, since not all devices will fit in the tables of all proxies. The results are high traffic and table entry overflows. Consequently, in the distributed case, it cannot be guaranteed that there is at least one proxy table entry per resource restricted device.